Sorbents are materials that are widely used for the capture of various small molecules, e.g., from gas streams. For example, nuclear power plants release radioactive iodine isotopes during the final oxidation of uranium and plutonium fission products. Iodine in cooled spent nuclear fuel consists of approximately a 3:1 isotopic ratio of 129I:127I. While 127I is a stable isotope, 129I emits beta and gamma radiation with a half-life of 10 million years. Due to its long half-life and extreme mobility in the environment 129I is likely to be the radionuclide with the greatest long-term environmental impact. The nuclear power plant waste streams generally contain, in addition to about 20-60 ppm iodine, 0.1% NOx gases, and high levels of water vapor. Accordingly, for disposal through nuclear transmutation or for long-term sequestration, selective and inexpensive capture materials are required.
Most sorbents generally used for gas capture are porous materials where selectivity is achieved through pore size discrimination, wherein cavities are matched in size with the guest molecules. In other approaches for gas capture, materials comprising pores with substrate-binding sites or flexible pore walls are utilized. See, e.g., L. J. Murray et al., Chem. Soc. Rev. 2009, 38, 1294-1314; and S. Horike et al., Nature Chem. 2009, 1, 695-704. In the particular context of iodine disposal/sequestration described above, porous silver-impregnated zeolites can trap iodine as silver iodide with a gravimetric capacity upper limit of about 33 wt %. See, e.g., B. R. Westphal et al., Mater. Res. Soc. Symp. Proc. 2010, 1265, AA02-04; and K. W. Chapman et al., J. Am. Chem. Soc. 2010, 132, 8897-8899. Due to their porosities, porous sorbents typically have relatively high volumes and are not desirable where sorbent space is advantageously minimized (e.g., in the case of iodine disposal/sequestration).
Non-porous materials have not been widely investigated for gas capture/storage applications due to the obvious lack of void space, although examples of nonporous coordination polymers and organic-molecular crystals that can store volatile molecules are known. See, e.g., G. Minguez Espallargas et al., J. Am. Chem. Soc. 2007, 129, 15606-15614; S. Libri et al., Angew. Chem. 2008, 120, 1717-1721; Angew. Chem. Int. Ed. 2008, 47, 1693-1697; C. J. Adams et al., Angew. Chem. 2007, 119, 1142-1146; Angew. Chem. Int. Ed. 2007, 46, 1124-1128; J. L. Atwood et al., Science 2002, 296, 2367-2369; J. L. Atwood et al., Science 2002, 298, 1000-1002; and J. L. Atwood et al., Angew. Chem. 2004, 116, 3008-3010; Angew. Chem., Int. Ed. 2004, 43, 2948-2950. However, such materials generally are not recognized as providing significant capacity for captured molecules.
It would be beneficial to provide non-porous structures capable of functioning as reversible and/or irreversible gas capture sorbents that function based on chemical reactivity with the gas(es) to be captured.